FCC report: White space devices work as “proof of concept”

The FCC's long-awaited report on white space devices finds that current …

The FCC's Office of Engineering Technology has released its long-awaited technical report (PDF) on "white space" prototype devices, and the conclusion comes right up front: "At this juncture, we believe that the burden of 'proof of concept' has been met. We are satisfied that spectrum sensing in combination with geo-location and database access techniques can be used to authorize equipment today under appropriate technical standards."

Note that focus on "proof of concept"; the OET doesn't argue that white space prototypes work flawlessly yet, and the report makes quite clear that they do not. In fact, they don't do lots of things, including "communicate with other devices." But what they do show is that spectrum sensing of TV signals can work with great accuracy, even if transmission currently causes problems.

The testing

Five devices were submitted for testing and came from Motorola, Microsoft, Philips, Adaptrum, and the Institute for Infocomm research. None were ready for production, and the devices showed tremendous variance when it came to details like detection speed; while Motorola's box could scan each TV channel for an existing signal in 0.1s, the Adaptrum device took a whopping 185s (per channel!).

But the devices were quite good at their main task in most of these tests: detecting TV signals. As the report notes, "All of the devices were able to reliably detect the presence of a clean DTV signal on a single channel at low levels in the range of -116 dBm to -126 dBm," which is tremendously sensitive. With "real-world" signals, though, that come complete with multipath distortions and other problems, the devices performed a bit less well.

Clean signal posed no problems

Real world signal was a little harder

In addition, some devices showed false positives and false negatives (which are the worrying ones here). That raises the possibility of having white space devices transmitting on occupied channels, which FCC tests confirmed would cause local interference. But the FCC appears to be operating under the assumption that geolocation databases would have to be consulted before devices could broadcast. Such databases (Google has offered to host one) would tell each device what TV channels are free in whatever region of the country the device is deployed; combined with spectrum sensing, geolocation should prevent even less-sensitive devices from broadcasting over occupied channels.

This was tested with the Motorola device. "During the field tests, the Motorola device’s geolocation/database feature was used in combination with its sensing capabilities," says the report. "In those tests, the Motorola device correctly reported all occupied channels used by stations within whose contours the WSD was operated."

In his press conference backing the white space idea earlier today, FCC Chair Kevin Martin indicated his support for allowing the devices to transmit in the next adjacent channels to TV signals (though at lower power than if buffer channels were available). This proposal, which was already trashed by the broadcasters, apparently comes from some OET testing. The OET report notes that "no interference was observed when the Adaptrum device transmitted on an immediate adjacent channel even with the transmitter in close proximityto the receiver with a roof-top antenna." (Adaptrum was one of the few devices that actually could transmit in prototype form.)

Cable problems

The cable industry has also been worried about white space devices, claiming that their broadcasts could affect cable systems. The OET found support for such "direct pickup interference" in some cases. To give you a sense of how such issues might affect some homes and not others, consider the following passages from the report:

1) "The cable converter (set-top) box was inserted into the signal path and connected to the DTV’s tuner input. The set-top box was tuned to virtual channel 220 (cable channel 77) and a good quality digital picture was observed. The Adaptrum transmitter was tuned to channel 26 and activated at full power (no external attenuation). Interference was immediately observed in the form of a complete loss of picture."

Yikes! But then, after replacing the cabling:

2) "Finally, all of the existing routing from the cable service wall outlet to the converter box (including amplifier, splitters and cables) was removed and replaced with a laboratory-grade patch connecting the wall outlet and the converter box input. The Adaptrum transmitter was reset to its maximum power (all external attenuation removed) and turned on. No interference was observed until the transmit antenna was moved very close to the converter box (within 0.3 meters)."

Not pretty, but it works

In other words, poor cabling and interconnects could lead to problems for digital and analog cable users. Sascha Meinrath of the New America Foundation told us earlier this year that the issue was no reason to delay on white spaces. "If cable installers have failed to properly terminate their lines, that would facilitate RF pickup. If providers have used such cheap gear that they are not properly shielded, that too would cause a problem," he said. "In either case, the failures lie with the cable industry—foregoing a useful new technological innovation (in essence, rewarding the profit-maximizing, corner-cutting that the cable industry alleges is a problem) doesn't make any sense whatsoever."

This may or may not be true, but the political reality is that the FCC has to tread carefully when it comes to certifying any new technology that has the potential to disrupt nearby cable systems. It's one thing to make an argument that installs should be done with better parts; it's another to explain to non-techie TV watchers why their signal is scrambled.

Balancing all the competing interests here is of course why the entire issue is as much political as technical, but the OET report and Kevin Martin's words today indicate that the idea will probably be approved in some form at the November 4th FCC meeting. Given the rough state of the current prototypes, we won't see anything workable on the market for some time, but it's at least beginning to look like the lengthy battle white space backers have waged for unlicensed access to spectrum may not have been in vain.

Given the nature of the OET findings, though, there's something in the report for every side to spin, and spin they will in the coming weeks as the vote nears.